Mode suppressor apparatus



Feb, 22, 1966 M. w. sT. CLAIR MODE SUPPRESSOR APPARATUS 3 Sheets-Sheet 1Original Filed Jan. 16, 1957 .,W Rm Et v5 mw K/ e .m w

r U M# Y B a 4 D A y m fn 6 7 C s O mm m w ATTORNEY Feb. 22, 1966 M. w.sr. CLAIR 3,237,131

MODE SUPPRESSOR APPARATUS Original Filed Jan. 16, 1957 3 Sheets-Sheet 2FREQUENCY ||.O

X DIA.

ioAol v 2 l RESONATOR 0.69 0.10 0,, LENGTH Portion of a CircularElectric Mode Char! INVENTORn Maur/ce W St. Cla/'r ATTORNEY Feb. 22,1966 M. w. sT. CLAIR 3,237,131

MODE SUPPRESSOR APPARATUS Original Filed Jan. 16, 195'? 28 Il 16M "L 3Sheets-Sheet 5 INVENTOR.

BY Maurice W Si. Cla/r ATTORNEY United States Patent O 3,237,131 MDESUPPRESSOR APPARATUS Maurice W. St. Clair, Menlo Park, Calif., assignorto Varian Associates, San Carlos, Calif., a corporation of CaliforniaOriginal application Jan. 16, 1957, Ser. No. 634,498, now Patent No.3,008,102, dated Nov. 7, 1961. Divided and this application Aug. 4,1961, Ser. No. 129,473 8 Claims. (Cl. S33-33) The present invention isan improvement relating to high frequency mode suppressor apparatus andis a divisional application divided out of a copending applicationentitled Cavity Resonator Method and Apparatus, Serial No. 634,498,tiled January 16, 1957, inventor Maurice W. St. Clair, now Patent Number3,008,102, issued November 7, 1961.

The present invention relates in general to a mode suppressor apparatusand, more specifically, to a low Q microwave mode suppressor apparatususeful, for example, as a mode suppressor in stabilizing cavityresonators, microwave filters and the like.

It is well known that a cavity resonator may simultaneously supportresonant fie-lds in a great m-any modes. The total field configurationwithin the cavity resonator comprises the superposition of all of theexcited electromagnetic fields of the oscillating modes. It is also wellknown that some dield configurations have less energy losses associatedwith them than other lrfield configurations. The energy loss associated`with a particular field configuration or mode is due to the 12R lossesattendant circulating currents in the cavity resonator walls.

One certain family of field configurations known as the circularelectric, or smoke ring family o'f modes or more precisely designated asthe Tlom,1 family wherein m and n may have `any integer values is knownto possess less loss associated .with its field than any other knownfamily of field configurations. Within this circular electric lfamily ofhigh Q modes there is one sub family having a higher Q than theremaining modes. This high Q sub family of modes comprises the higherorder TEM,n modes where n can have any integer value greater than 4. Ifa cavity resonator can Ibe made to oscillate in the higher order TEolnmodes exclusively, the cavity can be made to have an extremely high Q.The pro'blem resides in lbeing able to suppress the unwanted modes ofoscillation over a Wide range of frequencies for broad tuning rangewhile not lowering the Q of the desired modes.

Heretofore, various mode suppression schemes have been utilized forsuppressing unwanted transverse magnetic (TM) and transverse electric(TE) modes. In a co-pending application, entitled High [FrequencyT-unable Cavity Apparatus, Serial No. 480,207, filed January 6, 1955,now Patent No. 3,030,594, inventors Malcom L. Stitch et al., theunwanted TM family of modes, which are characterized by substantiallongitudinal currents in the cavity side walls, were suppressed 4lay theutilization of annular gaps between the resonator end conducting Wallsand side walls thereby preventing the flow of longitudinal currentsthere-between. These gaps suppressed all the unwanted transversemagnetic modes, Ihut there still remained a plurality of unsuppressediield configurations of the TE family which possessed considerably lowerQ than the ldesired circular electric family of modes and interferedwith continuous tuning of the cavity at points where they crossed thedesired mode.

Due to these unsuppressed relative high loss transverse electric modesit was possible to obtain only a 60 megacycle frequency range whereinthere were no interfering low Q modes within the cavity resonator whenoperating at X-band frequencies.

3,237,131 Patented Feb. 22, 1966 ICC The present invention providesnovel mode suppression and mode exciting techniques wherein theundesired transverse electric and transverse magnetic modes thatpreviously remained unsuppressed may Ibe suppressed such that the tuningrange of the high Q cavity resonator may be greatly enhanced to analmost unlimited extent. For example, at X-'b-and frequency rangeutilizing the novel features of the present invention it has been foundthat a cavity resonator `can be made having a tuning range of 2,000megacycles as compared to the previously obtainable 60 megacycle range.

An exemplary cavity resonator operating at X-band frequency will bedescribed; however, the invention is not limited to such a -frequencyrange and is equally applica- Ible in general to cavity resonatorsoperating at frequencies spread throughout the microwave range. Some lofthe features that will he described herein as they are applied to thenovel cavity resonator are applicable in general to Wave propagatingstructures and not limited exclusively to cavity resonator applications.It is not intended that the scope Vof the present invention should belimited to cavity resonators.

The principal object of the present invention is to provide a novelimproved mode suppressor apparatus for suppressing wave energy coupledthereto and being useful in high Q resonant circuits such as, lforexample, high frequency stabilizing cavity resonators, microwaveLfilters and the like.

One feature of the present invention is the provision of a novel waveenergy attenuator comprising a resonant iris opening into a capacitivelyloaded and lossy waveguide whereby energy coupled into the loadedwaveguide via the resonant iris may `be heavily attenuated.

Another :feature of the present invention is the same as the precedingfeature including in combination a cavity resonator having one or moreof such attenuators communicating with the fields of the resonatorthrough one or more end walls of the resonator for suppressing certainundesired modes.

Other features of the present invention will become yapparent on aperusal of the specification taken in connection with the accompanyingdrawings wherein,

FIG. 1 is -a side elevational view, partly in section showing the novelcavity resonator of the present invention.

FIG. 2a is an enlarged fragmentary view of a portion of the structure ofFIG. l delineated -by line 2-2 of FIG. 1 and depicting the method `offabricating the side walls of the structure of FIG. 1,

FIG. 2b is an enlarged fragmentary view of the structure of FIG. 1delineated by line 2-2 of FIG.l,

FIG. 3 is a portion of a circular electric mode chart depicting thetypical modes of operation within a limited frequency range of thestructure o-f FIG. 1,

FIG. 4 is a schematic drawing showing the TE0Y15 circular electric modeconfiguration,

FIG. 5 is a plan view in schematic form of the current distribution ofthe TEOL mode in the end plates of a circular cavity resonator,

FIG. 6 is a diagrammatic plan view of an end plate of cavity resonatorshowing the TElan mode current distribution,

FIG. 7 is an enlarged cross-sectional view of a portion of the structureof FIG. 1 taken along line 7 7 in the direction of the arrows,

FIG. 7a is -an enlarged fragmentary view of a portion of the structureo-f FIG. l delineated by line 7a-7a of FIG. 1,

FIG. 8 is an enlarged view of the portion of the structure of FIG. 7ataken along line 8-8 in the direction of the arrows,

FIG. 9 is an enlarged view of the portion of the structure of FIG. 7utaken along line 9-9 in the direction of the arrows,

FIG. is a reduced view of the structure of FIG. 8 showing two of thedevices of FIG. 8 disposed in right angle relation with respect to eachother as lfound in the structure of FIG. 1,

FIG. 11 is a diagrammatic view of the fields of the TEOzJ, mode in acylindrical cavity resonator, and

FIG. 12 is a diagrammatic view of the end Wall of the cylindrical cavityresonator showing the current density distribution for the TEM,n mode.

Referring now to FIG. 1 a cavity resonator 1 is depicted having its sidewalls defined by a cylindrical conducting liner 2 which will be morefully described later in the specification. The liner 2 is carriedwithin a hollow bell-shaped cavity housing 3 having longitudinal ribs 4thereon which is made of magnesium so as to be lightweight and rigid.One end of the cavity resonator is defined by a moveable transversecircular plunger 5 which is translatable along the longitudinal axis ofthe cavity resonator 1. The other end of the cavity resonator is definedAby a rigid transverse circular end wall 6 fixedly secured to the cavityhousing 3. The cavity housing 3 is provided with a flange 7 at the wideend thereof which is suitably bored around its periphery and anchored inshock resistant mounts 8 via hold down cap screws 9. The shock resistantmounting of the cavity resonator 1 prevents unwanted shock and vibrationfrom being transmitted from the environment to the cavity resonator andthereby reduces unwanted microphonic frequency perturbation in operationof the resonator.

A tuner assembly is mounted over the cavity housing 3 at the wide endthereof and serves to provide means for translating the tuning plunger 5within the cavity resonator 1 to thereby tune the frequency of theresonator. An indicator 12 is provided for indicating the position ofthe plunger 5 within the cavity resonator and may be calibrated in termsof frequency or plunger position, as desired. Two tapered rings 13 and14 are provided between the tuner assembly V11 and the anged portion 7of the bell-shaped cavity housing 3 for providing alignment of thetuning plunger 5 within the cavity resonator 1. The tuning assembly 11,tapered rings '13 and 14 and frequency indicator 12 are more fullydescribed in the aforementioned copending application.

Since one of the primary uses :of a cavity resonator, as shown in FIG.l, comprises its use as `a stabilizing resonator for stabilizing theoscillating frequency of a high Ifrequency oscillator, the cavityresonator 1 is shown as it would -be utilized for this application.Accordingly, a high frequency :oscillator 15 such as, for exampleyareflex klystron is shown coupled to the cavity resonator 1 via a shortlength of rectangular waveguide 16 which includes a portion of itslength milled out of the cavity end plate 6. The rectangular waveguide16 couples to the cavity resonator 1'via-a circular coupling iris 17(see FIG. 7).

A coupling screw 18 extends into lthe waveguide 16 in close proximity tothe coupling iris 17 and is provided for varying the coupling betweenthe oscillator 15 and the cavity resonator 1, as desired. In addition aphase control screw 19 is provided extending into the short.

length of waveguide 16 for -adjustably changing the electrical length ofthe transmission line 1oetween the oscillator and the cavity, asdesired, for maximum stabilization.

As a result of hav-ing a length of transmission line between the reflexklystron oscillator 15 and the cavity resonator 1 two transmission lineresonant modes are introduced, one on the low and one on the highfrequency side of the desired resonant frequency of the cavityresonator. These unwanted modes occur at a frequency such that thesusceptance of the cavity is equal and opposite to the susceptance ofthe cavity is equal and opposite to the susceptance of the electricallylengthened or shortened transmission line loaded at the end by the offresonance impedence of the cavity. A thin card of lossy material 20 aslof, for example, mico coated with a platinum film undesired modes ofoscillation.

is placed within the section of waveguide such that the thin edge ispresented to the direction of the 'propagation of the wave energy andthe plane of the card lies in the region of maximum electric field ofthe undesired modes, approximately one-quarter wavelength from thecoupling iris 17 whereby the undesired resonant modes of thetransmission line are heavily attenuated and thereby effectivelysuppressed. In the absence of suppression of these waveguide modes thecavity resonator is not a dependably self-starting device.

A circular output iris 21 is provided in the end plate 6 for couplingwave energy out of the cavityresonator 1 via a short section ofrectangular waveguide 22 to a load 23. A coupling screw 21% is providedextending nito close proximity to the output coupling iris 21 forcontrolling the wave energy coupling to the load 23, as desired.

Although the coupling irises 17 and 21 have been described and depictedas circular irises, they need not be of circular shape and could bereplaced by other and different suitable coupling means such as, forexample, coupling loops which are well known in the art.

Two mode Suppressors 26 are provided in the center of the cavity endwalls Siand 6 for suppressing certain The mode Suppressors 26 will bemore fully described later in the specification.

In operation, the output signal from oscillator 15 is fed via waveguide16 and the coupling iris 17 to the cavity resonator 1. The cavityresonator, due to its close electromagnetic coupling to the oscillator15, serves to stabilize the frequency of the oscillator at the frequencyof the exciter and coupled to resonant mode of the cavity resonator 1. Aportion of the stabilized RF. signal is coupled fromthe cavity resonator1 through the output coupling iris 21 via the output waveguide 22 to theload 23. A stabilizing cavity resonator built according to the teachingsof the present specification has provided an effective Q ofapproximately 120,000 and a stability factor of betwen and 200 over thefrequency range of between 8 to 10 kilomegacycles.

The degree of `stabilization produced by a given cavity resonator variesdirectly as the Q of the resonator. Therefore, to achieve high stabilityfactors it is necessary that the cavity resonator have an extremely highQ. As was pointed out previously, if the cavity resonator can be ma-deto operate exclusively on the Tloyl,n mode of the smoke ring modefamily, the highest Q resonant mode would be provided. For the purposesof illustration, one of the desired higher order smoke ring modesdesignated as the TE015 is schematically depicted in FIG. 4 of the`drawings and is characterized by'a plurality of single transverseannular electric field configurations spaced apart in axial alignment.

To operate exclusively on the desired smoke ring mode all otherinterferingV modes of oscillation within the frequency range of thecavity resonator must be effectively suppressed. It turns out that allmodes of oscillation except the circular electric field modes (smokering modes) require at 'least ysome component of current to flowlongitudinally in the side walls of the cavity resonator to support theelectromagnetic fields thereof. Included in the modes requiringsubstantial longitudinal currents in the side walls are the transversemagnetic (TM) modes and many transverse electric (TE) modes.

The transverse magneticy (TM) modes are effectively suppressed, as inthe copending application, bythe provision of annular' gaps 27 (FIGS. 1and 7) at the junctions of the end walls and the side walls of thecylindrical cavity resonator cooperating with lossy material 2S (seeFIG. 7) disposed in the side wall of the gap. It also turns out thatmany of thetransverse electric modes of oscillation havel at least `someappreciable longitudinal current fiowing in the side walls of the cavityresonator. Some of these modes are not effectively suppressed by theprovision 'of the .annular gaps at either end of the cavity resonator. l

The present invention provides novel methods and apparatus forsuppressing all of the unwanted low Q transverse electric modes ofoscillation while not appreciably lowering the Q or suppressing thedesired high Q smoke ring family of modes. The novel method includes theprovision of alternately spaced relatively wide transverse conductingsurfaces spaced apart by relatively thin transverse lossy elementsdisposed in longitudinal spaced apart relation and defining the cavityresonator side wall. This method has proven quite satisfactory forsuppressing the large majority of unwanted modes without appreciablyeffecting the desired circular electric yfamily of modes.

In a preferred embodiment of this method of mode suppression, the cavityresonator is fabricated in a novel way (see FIGS. 1, 2a and 2b) whichcomprises the steps of boring the cavity housing 3 to an approximatediameter and depth and then coating the interior surfaces of the boredcavity housing with a layer of lossy adhesive material 29 (see FIGS. 2aand 2b) such as, for example, epoxy resin. Such an adhive substance maycomprise, for example: 10.5 parts by weight of Shell curing agent D to100 parts by weight of Shell No. 828 epoxy resin to 500 parts by weightof carbonyl iron, grade E made by the'General Aniline & Film Corp. Thethickness of the adhesive substance 29 is approximately 50 thousandthsof an inch.

While the lossy adhesive substance 29 remains fluid, an electricalconductor 31 as of, for example, copper is tightly wound in a helicalconfiguration within the interior of the bore, the longitudinal axis ofthe spiral coinciding with the longitudinal axis of the bore. (See FIG.2a.) The helically wound conductor 31 may comprise, for example,standard enameled wire such as, for example, Formvar No. 16 wire made bythe General Cable Co. Although a standard enameled Wire was cited in theexample, the conductor 31 need not be of circular cross section norcoated.

The free ends of the tightly Wound conductor 31 are iirmly held as byclamping to the cavity housing 3. The cavity housing 3 is then insertedinto an oven and cured for 2 hours at a temperature of approximately 65C. After curing the spirally wound conductor 31 is firmly embedded inthe lossy epoxy resin which in turn is firmly bonded to the magnesiumcavity housing 3. The clamps are then removed from the cavity housing 3and the cavity housing 3 chucked in a lathe.

Successive cuts are then taken along the inside surface of the helicalconductor 31. Each successive cut increases the inside diameter of thehelical winding. The inside diameter of the helical winding is increaseduntil the helical cond-uctor 31, if of a circular cross section, is cutdown to the diameter of the wire. At this inside diameter of the helicalwinding, the conductor 31 will present a maximum of conductive surfaceto the side wall of the cavity resonator defined thereby and a minimumsurface area of lossy material. The lossy surface area is delined by thevery thin layer of lossy adhesive 29 disposed between adjacent turns ofthe helically wound conductor 31. When No. 16 gage enameled wire is usedfor the helical conductor 31 the thickness of the alternately spacedconductors at the surface of the cavity resonator are approximately50-thousandths of an inch in width separated by 4-thousandths of an inchof enamel and approximately l-thousandth of an inch of lossy material(see FIG. 2b).

A cavity resonator having its side walls constructed of a tightly woundhelical conducting material separated at adjacent turns by a thin layerof lossy material, as shown in FIGS. l and 2b, will satisfactorilysuppress practically all unwanted modes of oscillation which arecharacterized -by longitudinal currents in the cavity side walls. Inaddition, the helical conducting surface has an almost negligibleloading effect upon the desired circular electric family of modes.

One undesired family of modes, the TElgln family, is not sufficientlysuppressed by the helical conductor technique. The reason for this isthat the longitudinal cornponents of current flowing in the side wallsof the resonator necessary to support the fields of this mode are verysmall. Fortunately, however, this TEM,n mode has heavy currentconcentration in the center of the end plates where the currentconcentration `of the desired TEOLn mode of the smoke ring family ofmodes approaches zero. These relative current concentrations can readilybe seen by reference to FIGS. 5 and 6 wherein the current concentrationsfor the TEOLn family and the TE13n family of modes are shownrespectively.

The novel mode suppressor 26, previously mentioned, is positioned at thecenter point of the cavity end walls 5 and 6 where the currents of theundesired TEM,n modes are a maximum and they serve to suppress theunwanted T ELM modes. Essentially, the mode suppressor 26 comprises ashort section of cylindrical waveguide coupled to the cavity resonator 1via a resonant iris and has its interior capacitively loaded with alossy dielectric material for absorbing energy propagated therethrough,and for increasing the cut-olf Wavelength to a point which allows suchpropagation.

Referring now to FIGS. 7 through 10 the novel mode suppressor 26 will bedescribed in greater detail. The end wall of the cavity resonator iscentrally bored to receive a hollow cylindrical conductor 33 forming thecylindrical waveguide section. The receiving end of the waveguide isclosed by a transverse conducting wall 34. The interior of the shortsection of cylindrical waveguide 33 is filled with a lossy material 35such as, for example, epoxy resin having the following proportions: 10.5parts by weight of Shell curing agent D to 100 parts by weight of ShellNo. 828 epoxy resin to 200 parts by weight of carbonyl iron grade E. Twospaced apart longitudinal bores 36 and 37 are provided in the epoxyresin to produce the desired characteristic impedance of the shortlength of transmission line.

A dumbbell types resonant iris 38 is provided closing olf the other endof the short section of cylindrical waveguide and serves as a low Qresonant iris for coupling energy from the cavity resonator 1 to theshort length of cylindrical waveguide. A relatively short space 39 isprovided between the resonant iris 38 and the lossy dielectric loadingsubstance 35 of the waveguide for providing an impedance match betweenthe resonant coupling iris 38 and the loaded waveguide.

Two such mode Suppressors 26 are provided for suppressing the TELSJ,family of modes, one in the center of each end conducting wall of thecavity resonator. The two mode supressors 26 disposed at either end ofthe cylindrical cavity resonator 1 have the longitudinal axes of theirdumbbell irises disposed substantially with respect to each other asshown in FIG. l0. The reason for this 90 relative orientation canreadily be seen by reference to FIG. 6 wherein the current distributionand direction for the TELgn mode are shown as they would appear in theend walls of the cylindrical cavity resonator. Notice that in the centerof the end wall in the vicinity of the mode suppressor 26 the currentsassume a rectilinear motion and have a certain direction depending uponthe particular orientation of the TEM,n mode. The wave energy couplingbetween the rnode suppressor 26 and energy of the TElan mode is amaximum when the currents of the TE 1331 mode in the vicinity of themode suppressor are substantially at right angles to the longitudinalaxis of the dumbbell iris 38. The coupling between the currents of theTElan mode and the mode suppressor 26 follow approximately a sinerelationship as a function of the angle between the longitudinal axis ofthe dumbbell iris 38 and the direction of the currents. Since the TEM,nmode may assume any one of an infinite number of orientations within thecavity resonator 1 disposing the axes of the mode Suppressors 26 atright angles to each other assures close coupling to the TElgyn mode atall times and for all possible mode orientations.

The entire circular electric family of modes (TEOYnm), containing boththe wanted and unwanted sub-families of modes, will be supported by theotherwise mode suppressed cavity resonator thus far described. Referringnow to FIG. 3 there is shown a circular electric mode chart describingthe excitable modes in a certain cylindrical cavity resonator exemplaryof a resonator as previously described. The heavy vertical line on themode chart indicates the maximum extent of plunger travel. The left handordinate ldefines the other extent of plunger travel. It can be seenthat the undesired TEM,n mode interferes with the desired TEM),n modewthin the tuning range of the cavity resonator.

Referring now to FIG. ll there is shown in diagrammatic form the eldconlguration for the interfering and unwanted TEoyzn mode as it appearsnear the end wall of the cavity resonator. It can be seen that theunwanted TEM,n mode has an electric field configuration characterized bytwo circular electric field rings concentrically disposed. The electriceld vectors in the inner and outer concentric rings are found to be inopposite directions.

Referring now to FIG. 12 there is Ashown diagrammatically the currentdistribution in the cavity end wall for the unwanted and interferingTEM,n mode. It can be seen from this diagram that at a certain radiusfrom the center of the cavity end wall there is a minimum currentdensity for circulating currents of the TEM,n mode. In FIG. there isshown a similar diagram for the current densities of the circulatingcurrents for the desired TEOJJ, mode. Comparing these two diagrams itcan be seen that the TEoln mode has a maximum Acurrent density near aradius where t-he current density of the TEOQJ, mode is a minimum. Bylocating the input and output coupling irises 17 and 21 respectively atthis certain radius from the center of the cavity end wall, the iriseswill couple stronglyto the desired TEOLI, mode and provide negligiblecoupling to the unwanted TEM,n mode. This being the case the TEM,n modeis not excited and for practical purposes acts as though it weresuppressed.

Referring now to the circular electric mode chart FIG. 3 and consideringthe TEM.n mode to be suppressed, it can be seen that the desired pureTEOM, modal operation is achieved over a wide range of frequencies. In acertain exemplary cavity resonator whch has been built to operate atX-band frequency this range of pure TEOln modal operation exceeds 2,000megacycles.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A mode supression apparatus for suppressing undesired wave energycoupled thereto including, a waveguide, attenuating means disposedwithin said waveguide, a low Q resonant iris for coupling wave energyinto said waveguide to be attenuated in said attenuating means withinsaid waveguide, and said attenuating means serving to capacitively loadsaid waveguide for impedance matching said loaded waveguide to said waveenergy coupled thereto, said capacity loading means including adielectric lossy material substantially filling said waveguide andhaving an axial bore therein for matching said loaded waveguide to saidiris.

2. The apparatus according to claim 1 wherein said dielectric materialincludes, a second axial bore spaced from said rst axial bore, and saidiris being dumbbell shaped.

3. The apparatus according to claim 2 wherein said dielectric materialis spaced apart from said dumbbell iris.

4. Mode suppressor apparatus for suppressing undesired modes ofoscillation within a wave propagating structure including, a waveguide,a conductive common wall separating said waveguide from the wavepropagating structure, a coupling iris in said common conductive wallmember for electromagnetically connecting the wave propagated structureand said waveguide for coupling wave energy to be attenuated from thewave propagating structure to said waveguide, a dielectric lossy membersubstanitally filling the cross sectional area of said waveguide forcapactively loading said waveguide and for suppressing wave energypropagating therein, said lossy dielectric member having an axiallydirected bore therein for impedance matching said capacitively loadedwaveguide to said iris.

5. The apparttus according to claim 4'wherein said iris is dumbbellshaped having spaced enlarged openings interconnected by a relativelynarrow slot, and said lossy dielectric member includes a second axiallydirected bore.

6. The apparatus according to claim 5 wherein said axial bores in saidlossy dielectric member are disposed in substantial registry with theenlarged opening portions of said dumbbell iris.

7. The apparatus according to claim 6 wherein said waveguide is a lengthof cylindrical waveguide.

8. The apparatus according to claim 6 wherein said dielectric member isspaced apart from said common conductive wall through which said iris isprovided.

References Cited by the Examiner UNITED STATES PATENTS 2,439,388 4/1948Hansen 333-83 2,593,234 4/1952 Wilson 333--83 3,008,102 11/1961 St.Clair 33-83 X ELI LIEBERMAN, Acting Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

1. A MODE SUPRESSION APPARATUS FOR SUPPRESSING UNDESIRED WAVE ENERGYCOUPLED THERETO INCLUDING, A WAVEGUIDE, ATTENUATING MEANS DISPOSEDWITHIN SAID WAVEGUIDE, A LOW Q RESONANT IRIS FOR COUPLING WAVE ENERGYINTO SAID WAVEGUIDE TO BE ATTENUATED IN SAID ATTENUATING MEANS WITHINSAID WAVEGUIDE, AND SAID ATTENUATING MEANS SERVING TO CAPACITIVELY LOADSAID WAVEGUIDE FOR IMPEDANCE MATCHING SAID LOADED WAVEGUIDE TO SAID WAVEENERGY COUPLED THERETO, SAID CAPACITY LOADING MEANS INCLUDING ADIELECTRIC LOSSY MATERIAL SUBSTNATIALLY FILLING SAID WAVEGUIDE ANDHAVING AN AXIAL BORE THEREIN FOR MATCHING SAID LOADED WAVEGUIDE TO SAIDIRIS.